Method and apparatus for screening pulp



Jan. 22, 1963 H. F. SZEPAN ETAL 3,074,553

METHOD AND APPARATUS FOR SCREENING PULP 4 Sheets-Sheet 1 Filed April 14, 1960 aukuuwum owhmmu N mm h INVENTORS. HENRY F SZEPAN,4I:J

BY g 5 4 /74/11 Jan. 22, 1963 H. F. SZEPAN ETAL 3,074,553

METHOD AND APPARATUS FOR SCREENING PULP Filed April 14, 1960 4 Sheets-Sheet 2 Jan. 22, 1963 H. F. SZEPAN ETAL 3,074,553

METHOD AND APPARATUS FOR SCREENING PULP 4 Sheets-Sheet 3 Filed April 14, 1960 INVENTORS. HENRY F. szEPAN J BY a, E. J'qnlfof-J Jan. 22, 1963 H. F. SZEPAN ETAL 3,074,553

METHOD AND APPARATUS FOR SCREENING PULP Fild April 14. 1960 4 Sheets-Sheet 4 INVENTORSZ HENRY F. SZEPAN,4I\J

(AITORNEY 3,074,553 METHOD AND APPARATUS FOR SCREENING PULP Henry F. Szepan, 295 Amherst St., Manchester, N.H., and Guy E. Sanford, 1070 Ibis Road, Jacksonville, Fla. Filed Apr. 14, 1960, Ser. No. 22,236 12 Claims. (Cl. 209273) It is an object of this invention to provide an improved method of screening pulp, particularly the hot stock as it comes from the digester.

It is a further object of this invention to screen hot alkaline stock without the need of foam breakers or foam eliminators.

It is a further object of this invention to provide an improved apparatus for carrying out said method.

It is a further object of this invention to provide an apparatus as aforesaid which will refine the rejects mechanically.

The above and other objects will be made clear from the following detailed description taken in connection with the annexed drawings in which:

FIGURE 1 is a schematic diagram of a complete installation with the preferred instrumentation indicated:

FIGURE 2 is a central section through a preferred form of screen;

FIGURE 3 is a sectional view of a preferred form of cover plate;

FIGURE 4 is an isometric projection of the gasket as a whole;

FIGURE 5 is a partial section of the tailings end of a modified machine; and

FIGURE 6 is a partial end elevation of the machine of FIGURE 5, showing the joint and severa means of exit of the tailings.

Reference is hereby made to the patent to Bowen, No. 2,845,848 dated May 8, 1958 and to the patent to Rich et al., No. 2,908,390 dated October 13, 1959 and to a pending application Serial No. 824,732, filed July 2, 1959, now Patent No. 3,034,650 dated May 15, 1962. With, of course, some physical alteration, the present invention may be adapted to any of these and, on the average, will increase the capacity of each by about fifty percent in air dry tons per 24 hours.

For general background on the art of pulp screening, reference is made to vol. I of Pulp and Paper Manufacture dealing with Preparation and Treatment of Wood Pulp (McGraw-Hill Book Company, Inc., 1950) and to pages 694-799 of that volume. The screening of pulp is one of the least predictable operations in an industry which even today is still more art than science. Direct observation is impossible, not merely because the material of the screens is opaque, but because the slurries being handled are equally opaque. Moreover, the solids content of the slur-ries renders pitot tube exploration difiicult and unreliable. In short, the facts are whatever they turn out to be, and the theory is merely an accessory to the facts.

The present invention is based on hermetically sealing pulp screens of the types mentioned so that the rotor will generate subatmospheric pressure on the interior of the screen, with the absolute pressure of the interior being less than the absolute pressure of the exterior. The absolute interior pressure seems to be the controlling factor. The absolute exterior pressure, to say the least, is not critical. Within reasonable limits even a positive superatmospheric absolute exterior pressure may occur. The exterior subat-mospheric absolute pressure is caused, in all probability, by leak-age across the screen plates, most probably at the rejects or tailings, end. Under such conditions, there is no foaming, and thus it is possible to handle pulps without the addition of antifoaming agents.

States act artists This makes it possible for a single machine .to act both as a knotter and as a coarse screen. Traditionally, about the only ditference between a knotter and a coarse screen is the difference in size of the perforations in the screen, and this again is the major difference between a coarse screen and a fine screen. It also must be emphasized that while this invention has special or extra value as applied to the digester-type slurry of hot pulp in hot, spent cooking liquor, it also presents considerable advantage (50 percent increased capacity, for example) in knotting, or screening any pulp suspended in any liquid. Thus the inclusion of washed pulp, that is, pulp from which, at some previous operation, some or all of its chemical content has been removed (washed) does not at all affect the applicability of this invention. In fact, since, in practice, there is no such thing as an absolutely non-foaming pulp slurry of any sort, the vacuum principle of this invention has value (in addition to increased capacity, screen for screen) in the deaeration, or at minimum, non-increase of aeration of the slurry at any location between the chipbin and the paper reel.

The term rejects and tailings are sometimes used synonymously, but are not quite synonymous. Tailings would refer to the total discharge from the interior of a screen, while rejects would refer to the solids content of the tailings. Even the term rejects, as just defined, requires some qualification. Ordinarily, this term means such knots and shives as would be intolerable in the finished product plus, of course, some unavoidably entrapped good fibre. If, however, the infeed slurry is a mixture of reclaimed groundwood (newsprint) and kraft, for example, the rejects would be predominantly kraft fibres and the accepted stock would be predominantly groundwood. In such case the screen is acting as a fibre classifier.

FIGURE 1 is generally illustrative of a preferred installation. In an actual installation, the details will vary from plant to plant depending on the pulp and on the physical limitations imposed by the location. In FIGURE 1 there is shown a brown stock blow tank 10 at the bottom of which, optionally, is a mechanical attrition mill 12 which may be of any of several tor-ms presently available. This mill 12, if used at all, receives pulp from the tank 10, reduces the oversize particles, and 'also acts as a pump to feed the treated pulp through a line 14 and through a flow meter 16 to a pair of lines 18 and 20, each of which feeds primary screens 22 and 24. Accepted stock from screen 22 goes through line 26 to a line 28 while accepted stock from the screen 24 goes to line 30 through a line 28. Rejects from screen 22 pass through a line 32 to a pump 34, while reject-s from screen 24 pass through a line 36- to the pump 34. The pump 34 delivers the combined rejects of screens 22 and 24 through a line 38 to a line 40 which is connected to the original supply line 14 and which feeds a secondary screen 42. Accepted stock from the secondary screen 42 passes through a line 44 to the line 28 which carries acceptable stock from all three screens to a conventional pulp washer 46.

Rejects from the secondary screen 42 go to a pump 48, thence through a line 50 to a rejects screen 52. Here the rejects are deliquored and freed from substantially all good fibre. The rejects from this screen 52 are returned to the chipbin through a line 53. The liquor and good fibre pass through a line 54 back to the blow tank 10.

With the arrangement just described, the pulp slurry comes to the blow tank direct from the digester at a temperature usually about F. For any particular pulp and screen combination there is usually a consistency at which optimum performance is obtained. This is attained by dilution in the individual feed line of each screen. It is important that supply coming from the blow tank through the line 14 be uniform in consistency. For this reason a consistency regulator 56 is connected to the line 14 through a line 58, and through a consistency controller 60 adjusts a valve 62 which is connected to a black liquor line 64 which in turn is supplied by a dilution pump 68 connected to a primary liquor tank 66. The tank 66 receives its liquor from the washer v 46 through a line 67. A recorder controller 70 responds to the flow meter 16 to control the total flow through the line 14 by means of a recirculation valve 71, which connects with the consistency regulator 56 to recirculate excessive flow.

The instrumentations of the three screens is the same for each and only one, that for screen 24 will be described. As previously noted, the screen 24 is supplied with stock by a line 20. A pump 72 draws black liquor from the. tank 66 and through a line 74 and a branch line 76 delivers the black liquor to the screens as redilution liquor and also, through a line 78 connects with the. stock feed line 20. A valve 80 is placed in the line 78 and a valve 82 is placed in the line 20. The valve 80 and the valve 82 are actuated by a pressure controller 84 which responds through a line 86 to the absolute internal pressure within the screen 24.

For reasons which will be made clear hereinafter, the interior of the screen operates at a subatmospheric absolute pressure approximately 8" Hg. minimum absolute pressure depends on the temperature of the slurry. In general the lower the temperature, the lower the absolute pressure. There are three functions accomplished by the controls just discussed. First, they assure that the screens will operate at a maximum consistency for cleanliness, maximum capacity, and minimum H.P. Second, they permit the screens to adjust to stock conditions without affecting stock flow as controlled by the flow recorder controller 70. Third, they provide an automatic cycle which, in the case of a single screen plugging, will quickly clean the screen and restore normal operations without affecting flow from the blow tank 10.

As noted above, the 8" Hg absolute pressure in the interior of the screen 24 is believed to be optimum for hot stock, though this has to be determined empirically for each installation. recorder controller 84 senses a decrease in absolute pressure, it will actuate the value 80 to reduce the rate of addition of black liquor diluent. If, on the other hand, the blow tank consistency swings heavy, there will be an increase in absolute pressure, and the valve 80 will be opened to increase the dilution. If for some reason the screen 24 should continue to plug even after the valve 80 is fully opened, the interior absolute pressure will continue to increase even to positive superatrnospheric presure. The pressure recorder controller 84 is set to close the stock valve 82, while leaving the valve 80 wide open. This runs the screen on liquor alone and such running very quickly cleans the screen. With a clean screen, the pressure returns to normal, and the valves 80 and 82 are reset. This cleaning cycle usually takes no more than or seconds. As a result, a single screen can plug and clean itself without noticeably affecting the stock flow through line 14.

The machine for performing the subatmospheric screening of pulp is best shown in FIGURE 2. While the machine is. designed and intended to and almost always will operate at an internal subatmospheric absolute pressure, it is not inconceivable that, perhaps due to plugging or control failure of some sort, there may be attained, within the machine, a positive or superatrnospheric pressure. With the stock at or about room temperature, should such pressure burst the shell of the machine, the result would be unfortunate. In handling hot, brown'stock, however, at about 190 F., the

If, through the line 86 the pressure result could and would be disastrous. For this reason the shell of the machine is designed as a pressure vessel to resist internal pressure of at least 30 p.s.i. Fortunately, the same mechanical characteristics which enable the shell to withstand internal pressure equally enable it to withstand external pressure induced by the machine operating, as it must, when sealed oif from the atmosphere.

In FIGURE 2 there is shown a screen generally designated as made up of a convex head end 102, a cylindrical shell portion 104 and a concave end portion 106. Adjacent the convex head portion 102 where it is welded at 108 to the shell portion 104 is a bafile 110. The baffle 110 has one or more openings 112. The convex head 102 has an opening 114 by which it joins an inflow pipe 116.

A rotor shaft 118 penetrates the convex head 102, the concave head 106 and extends generally axially of the shell 104. It has a bearing 120 adjacent the convex head 102 and a bearing 122 adjacent the concave head 106. Keyed to the shaft 118 is a two-part bafile plate 124 of the sort disclosed in the patents and application above noted.

Rotor bars 126 are welded to the baflie plate 124 and are further supported by rings 128 to which they are welded in the manner set forth in the two cited patents. The portions of the bars 126 at the trailing side of the bafile 124 may or may not be pitched reversely, as noted in the pending application above identified.

A screen 130 surrounds the shaft 118 and is strengthened by ribs 132 which actually mark the joints between segmented sections of the screen. The screen 130 is perforated, preferably by circular perforations. The diameter and center to center pattern of the perforations will depend entirely on the nature of the screening operation and the nature of the pulp. Accepted pulp goes through the screen 130 and rejects (oversize) are retained and discharged at the concave end 106 through a rejects pipe 134. The accepted stock goes through a passage 136, welded at 138 to the shell 104.

The screen 130 bears at each end on a gasket supported by annular rings 146 and 146' respectively welded to the bathe 110 and to the concave head 106.

A water seal 140 with an inlet 142 and an outlet 144 is placed adjacent the bearings 120' and 122 and is separated from the interior of the shell 104 and the screen 130 by a stufiing 141 and is sealed on the other side by a stufling 143.

As thestock passes through the opening 112 it is caught up by the rotor blades 126 and flung centrifugally outward to form a pattern indicated by the dotted line 143. The slurry fills the space radially outward of the line 148 and a void is created radially inwardly of the line 148. Precise delineation of the line 148 is impossible as at present advised,.but, as shown in FIGURE 2, the line conforms to the best available evidence. Inside the line 148 there is a vacuum preferably approximating 8" Hg and controlled at about that point by means previously discussed. Outside the screen 130 there is a body of accepted stock 150 roughly delineated by the dotted line 152. The pressure outside the line 152 is an absolute of about 2" Hg below atmospheric pressure. This value is not at all critical. It will be realized that the rotor blades 126' act to pump the stock upstream from the 8" Hg absolute below atmospheric pressure of the screens interior to the 2" Hg absolute below atmospheric pressure of the screens exterior. These blades, as in the Bowen and Rich-Luthi patents, also act to feed the stock axially over the bafiie 124 after which the stock is subjected todilution through one or more pipes 156. This provides redilution of the pulp mass prior to rejection to give leaner rejects. In addition and as noted in the patcuts, the blades, on the trailing side, have a cavitation elfect tending to clean the screen perforations. The cleaning effect is enormously improved by the existence of the internal relative subatmospheric pressure.

There must, of course, be access to the interior of the shell 104. This is accomplished by means of the cover plate and gasket shown in FIGURES 3 and 4. The cover plate, like the shell itself, must be engineered to withstand from to Hg of vacuum, up to 30 P.S.I.G. Bolting or capscrewing such a plate would lead to so much down time for any servicing as to be prohibitive cost-wise. Accordingly, a sliding bayonet type joint has been devised. The sliding has been set up generally parallel to the axis of the rotor and is accomplished by means of axially spaced lugs 200 welded to the shell 104 adjacent a cut 202 in the shell 104 as seen in FIGURE 3. The cover plate 204- has its axial edges machined and cut away to form lugs 206 which mate with the lugs 200. The lugs 200 have a base portion 208 and a flange 210 to form a channel 212. In the channel 212 is placed a preformed, expansible rubber (or rubber-like, e.g.: neoprene tube) 2114 indicated in FIGURE .3. The tube 214 is inflatable to the dotted line position 215 as shown in that FIGURE. To accomplish this the tube 214 will usually be inflated to from 50 to 100 P.S.I.G. from the usual mill pressure line. The inflation is done from a pressure line 216 operating, at least through the bending stage through a steel shell 218 which is welded to the shell 104 at 220. The gasket as seen in FIGURE 4 is molded to the form shown and is inflatable as above noted.

FIGURES 5 and 6 show a modification in which the rejects are subjected to mechanical attrition for recirculation, or discard or both. So far as the parts are the same as FIGURE 2, the reference numerals are the same. In FIGURE 5 the rotor blades 126 are given an extra reinforcing ring 128 and have bolted extensions 129. A hub 300 is keyed to the shaft 118. A disc 302 is welded to the hub 300. The disc 302 is dished to conform to the concave shell end 106 and is axially slidable for adjustment. At intervals around the periphery of the disc 302 there are impeller blades 304. The blades 304 are pitched at about 15 to the axis of the shaft 118. The screen support ring 146 has welded thereto a baffle 306 dished to conform to the disc 302. The baffle 306 has an annular flange 303 which is perforated at 309. The baifle 306 loosely surrounds the hub 300.

The shell 106 has the usual dilution connection 156. In addition there is provided a tangential outlet 310 and an axial outlet 312. A ring 314 is perforated at 316 with the same size and pattern of perforations as the flange 308. The ring 314 is adjustable relative to the flange 308 by means of slots and bolts not shown. This permits a regulation of the rate of flow of the rejects through the perforated flange 308 of the disc 306.

In fact, the machine as above described take the rejected shives and knots and reduces or deflbres them between the bolted bars 129 and the perforated ring 308. Toothpick sized shives emerging from the rejects side of screens 22 and 24 of FIGURE 1 will undergo attrition between the blades 129 and the flange 308 (of FIGURE 5) and the remaining fragments will pass into the path of the impellers 304 on the disc 302.

Now, depending on the grade of product the pulp mill makes, it may be desirable to recirculate the stock advanced by the impellers 304 to the unscreened line feeding the same screen. If this is done, rejects would move from the exit 310, FIGURE 5 directly to a connection 320 in the inlet 115 of FIGURE 2. The inlet 116, being directly connected to the interior of the screen 130 is subject to the interior subatmospheric pressure of that screen. Accordingly, in most cases, no additional pumping action need be supplied to cause a flow of rejects from the exit 310 to the entry 314.

The rejects of course can be fed to a secondary screen which, in all probability would have smaller perforations than those of the screen from which the rejects come.

The rejects, of couse, are returnable to the blow tank 10 or to the screen 52. In either of the cases, however, booster pumping almost always will'be required.

The blades 304, as previously noted, are pitched about 15 and, on the side facing the ring 308 are relieved about 5 to provide, in effect, a cutting edge to sever the shives of narrow, elongated dimensions which might penetrate the perforations 316, even though they are unacceptable by normal standards. With a preferred eight blades 304 and a preferred R.P.M. of from 300 to 350, it is quite clear that attrition over and above that occurring between the bars 129 and the flange 308 will occur between the flange 308 and the blades 304.

If recirculation is desired, use is made of the tangential outlet 310. If discard is desired, use is made of the axial outlet 312, normally covered by a plate 315 and sealed by a gasket 316. The 15 pitch of the blades 304 feeds the rejects in such manner as to direct the rejects both axially and tangentially, henceeither or both of the exits 310 and 312 may be used. I As a matter of fact, the impeller blades 304 and the disc 302, depending on the nature of the rejects, or tailings, may not be required. The attrition between the extensions 129 and the ring 314 may be suflicient and, for some end-use products, undoubtedly is.

As previously noted, this matter of knotting or screening hot or cold, washed or' unwashed pulps is subject to rather numerous independent variables. Accordingly the provision of specific examples, individually, by permutations and combinations, is out of the question. The following example gives actual data for the performance of a typical hot stock screening operation. If it were a knotting operation the perforations would be larger and the R.P.M. less. It is believed that this example, which contrasts sharply with the prior art, will enable those skilled in the art to adapt this invention to other specific conditions and objectives.

Typical Performance of Hot Stock Vacuum Screen Process location of screen: after blow tank and ahead of brown stock washer.

Size of screen plate area 24 square feet. Stock Pine. Permanganate 28. R.p.m 306. Stock temperature F. Feed consistency 1.5%. Black liquor redilution 300. Percentage of rejects in feed stock 4.5%. Plate perforation 0.100". Accepted stock capacity, tons per day air dry 340. Accepted stock c0nsistency 1.36%. Tailings consistency Approximately 2.0%. Tailings, g.p.m Approximately 225 (of which 40% is good fibre and 60% rejects). Brake horsepower 90. Internal vacuum, Hg 10".

The detailed description heretofore given of course is susceptible of considerable modification and is to be limited only as set forth in the subjoined claims.

What is claimed is:

1. A method of screening pulp comprising: advancing a slurry of pulp axially along the interior of a cylindrical screen formed of perforated sheet stock; forcing accepted stock outwardly through the perforations of said screen; and maintaining a cavity in the stock in said screen at a lower absolute pressure than that surrounding the exterior of said screen.

2. A method according to claim 1 in which the slurry is a mixture of pulp and spent cooking liquor at blow tank temperature.

3. A method according to claim 1 in-Which the slurry includes washed pulp.

4. Apparatus for screening pulp comprising: a stationary cylindrical screen; a shell surrounding said screen, said shell having an inlet opening and outlet openings for accepted and rejected stock, said shell having pressure resistant ends; a multiple bladed rotor within said screen,

said rotor being secured to a shaft penetrating both ends of said shell and a vacuum sealed bearing at each end of said shaft, said bearings with said ends and said shell forming a sealed chamber at the interior of said screen, and means to maintain the interior of said screen at lower absolute pressure than the exterior of said screen.

5. Apparatus as set forth in claim 4 in which an annular, perforated bafile is placed adjacent the trailing end of said rotor and adjacent the blades thereof.

6. Apparatus as set forth in claim 5 in which defiberin'g elements adjacent the trailing side of said baffle are secured to said shaft for rotation therewith.

7. Apparatus as set forth in claim 6 in which the defibe'ring elements are pitched relative to the axis of said shaft so 'as to have a pumping action.

8. Apparatus as set forth in claim 7 in which the rejects end of the shell has a tangential outlet adjacent the defibering elements.

- 9. Apparatus as set forth in claim 7 in which the rej'ec't's end of the shell has an axial outlet adjacent the defibering elements.

10. Apparatus as set forth in claim 9 in which the rejects end of the shell has a tangential outlet adjacent the defibering elements.

11. A method of treating brown stock comprising: subjecting a slurry of freshly digested pulp in hot, spent cooking liquor to mechanical attrition; conducting the attritioned stock while in said liquor to a screen; maintaining a diflerential in absolute pressure on opposite sides of said screen; forcing acceptable stock through said screen against said pressure differential and thereafter subjecting said acceptable stock to a conventional brown 5 stock washing process.

12. A method as set forth in claim 11 in which absolute pressure on the input side of said screen is subatmospheric.

References Cited in the file of this patent UNITED STATES PATENTS 833,617 White Oct. 16, 1906 1,225,198 Westby May 8, 1917 1,260,320 Bulley Mar. 26, 1918 1,320,128 Felix Oct. 28, 1919 1,722,874 Wells July 30, 1924 2,363,188 McDorman Nov. 21, 1944 2,704,604 Ralston Mar. 22, 1955 2,705,562 Albertson Apr. 5, 1955 2,845,848 Bowen Aug. 5, 1958 2,908,390 Rich et al Oct. 13, 1959 

4. APPARATUS FOR SCREENING PULP COMPRISING: A STATIONARY CYLINDRICAL SCREEN; A SHELL SURROUNDING SAID SCREEN, SAID SHELL HAVING AN INLET OPENING AND OUTLET OPENINGS FOR ACCEPTED AND REJECTED STOCK, SAID SHELL HAVING PRESSURE RESISTANT ENDS; A MULTIPLE BLADED ROTOR WITHIN SAID SCREEN, SAID ROTOR BEING SECURED TO A SHAFT PENETRATING BOTH ENDS OF SAID SHELL AND A VACUUM SEALED BEARING AT EACH END 